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July 31, 2012 -- A new and virulent subtype of flu bug has emerged among harbor seals in New England, researchers report.

Over a four-month period beginning last September, 162 harbor seals were found dead or dying along the coast of New England. An investigation by renowned virus hunter W. Ian Lipkin, MD, of Columbia University, identified the killer: a mutant flu bug transmitted to the seals by sea birds.

Virus isolated from the seals had undergone a series of important mutations:

It became able to spread among mammals, or at least from seal to seal.

It became able to infect the seals' airways, destroying lung tissue or opening the door to fatal secondary infections.

It became more virulent.

"An additional concern is the potential [animal-to-human] threat that this virus poses, as it has already acquired mutations ... that are often, though not exclusively, regarded as prerequisites for pandemic spread," Lipkin and colleagues note in an article published in mBio, a journal of the American Society for Microbiology.

The basic type of flu virus isolated from the seals has long been known to science. It's called H3N8. An H3N8 virus that caused flu in horses entered the U.S. in the early 1960s. The canine H3N8 that in 2004 caused a flu outbreak among racing greyhounds apparently came from the horse H3N8. It now spreads easily among dogs.

The good news is that neither the equine H3N8 nor the canine H3N8 flu bugs infect humans. It's unlikely the seal H3N8 virus will spread directly to humans, either.

But Lipkin and colleagues suggest that seals, like pigs, may be able to harbor bird and mammal flu viruses at the same time. The deadly H5N1 bird flu is carried by sea birds. The worry is that mammal-adapted H3N8 will combine with H5N1, and a deadly virus will emerge.

That's only a remote possibility. Yet it's clear that H3N8 viruses already have learned to spread from birds to horses, dogs, and now to seals. And in a controversial experiment with ferrets, researchers have shown that H5N1 bird flu is capable of mutating into a form that spreads among mammals.

The warning from Lipkin carries weight among scientists. Lipkin directs Columbia University's Center for Infection and Immunity. He's also director of the World Health Organization's center on emerging diseases, and is co-chair of the National Biosurveillance Advisory Subcommittee for the CDC.